Swimming

Make a Swim check once per round while you are in the water. Success means you may swim at up to half your speed (as a full-round action) or at a quarter of your speed (as a move action). If you fail by 4 or less, you make no progress. If you fail by 5 or more, you go underwater and begin to drowning.

The DC for the Swim check depends on the water, as given on the table below.

Water

Swim DC

Calm water

10

Rough water

15

Stormy water

20*

*You can’t take 10 on a Swim check in stormy water, even if you aren’t otherwise being threatened or distracted.

Each hour that you swim, you must make a DC 20 Swim check or take 1d6 points of nonlethal damage from fatigue.
A creature with a swim speed can move through water at its indicated speed without making Swim checks and can always choose to take 10 on a Swim check, even if distracted or endangered when swimming. Such a creature can use the run action while swimming, provided that it swims in a straight line.

Drowning

Any character can hold her breath for a number of rounds equal to twice her Constitution score. If a character takes a standard or full-round action, the remaining time the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check every round in order to continue holding her breath. Each round, the DC increases by 1.
When the character fails her Constitution check, she begins to drown. In the first round, she falls unconscious (0 hp). In the following round, she drops to –1 hit points and is dying. In the third round, she drowns.
Unconscious characters begin making Constitution checks immediately upon being submerged (or upon becoming unconscious if the character was conscious when submerged). Once she fails a check, she drops to –1 hp (or loses 1 additional hit point, if her total is below –1). On the following round, she drowns.

Conditions

Disoriented (condition): The character has lost track of which way is up and therefore cannot account for buoyancy and is terribly off-balance. In addition to not being able to sacrifice swim speed to battle the effects of buoyancy or water currents, the character gains a -4 penalty on melee attack rolls and cannot use any ranged weapon. A spinning and flopping disoriented defender gains a +4 bonus to Armor Class against ranged attacks, but takes a -4 penalty to AC against melee attacks. Righting oneself is a move-equivalent action that provokes an attack of opportunity.
A character can, as a free action, choose to flop around so erratically that he causes himself to become disoriented. This would be a good way to avoid ranged attacks, as the target becomes harder to hit as its movement becomes chaotic and unpredictable.

Skills

Fly (underwater)

Creatures with a fly speed can add one quarter of their fly speed to their base swim speed with a successful Fly check, DC 15. They cannot add this speed in areas with a current
moving faster than 30 feet, or when they are disoriented. Having the ability to fly does not
confer the ability to move directly from sea to air.
One must first either crawl onto land, jump free from the water, or move to a position that is floating on top of the water. In lieu of an Acrobatics skill check to jump from the water,
any creature that is actively using its fly speed to enhance its swim speed may make a Fly skill check to launch itself into the air with the same DC that would be required for the Acrobatics skill check. Failure means that the character stops at the surface of the water, and may launch himself into the air on his next turn.